1. Field of the Invention
This invention relates to a process for steam reforming of hydrocarbons; particularly, to a process for steam reforming of highly desulfurized hydrocarbons.
2. Prior Arts
Steam reforming of hydrocarbons is a useful process for manufacturing industrial raw materials. A high-temperature reaction produces mainly hydrogen and carbon monoxide, and low-temperature reaction, mainly methane and carbon dioxide. Since the sulfur component in the hydrocarbons as raw materials poisons the steam reforming catalyst, the hydrocarbon is desulfurized before steam reforming.
So far, in a typical desulfurization process conducted prior to steam reforming of hydrocarbons, the organic sulfurs in the hydrocarbon are hydrogenated in the presence of a Ni-Mo or Co-Mo catalyst, and the produced H.sub.2 S is removed by adsorption to ZnO.
However, such a conventional method involves many problems. For example, if the hydrocarbon contains organic sulfurs, especially hardly decomposable organic sulfurs as thiophene in an amount higher than a certain level, in the hydrodesulurization step, undecomposed organic sulfur will slip and pass through without being adsorbed by the ZnO. Also, in adsorption desulfurization, for example, because of the equilibrium shown by EQU ZnO+H.sub.2 S.revreaction.ZnS+H.sub.2 O EQU ZnO+COS .revreaction.ZnS+CO.sub.2
the quantity of H.sub.2 S and COS is not decreased to less than a certain quantity. Particularly, in the presence of H.sub.2 O and CO.sub.2, this trend is remarkable. Further, if the desulfurization system is unstable in startup and shutdown of the plant, sulfur can be scattered from the hydrodesulfurization unit and adsorption desulfurization catalyst and increase the sulfur concentration in the refined product. Therefore, the desulfurization step in the steam reforming process at present must be controlled so that the sulfur concentration in the hydrocarbon after purification is in the level of several ppm to 0.1 ppm.
The thus desulfurized hydrocarbon is then subjected to steam reforming in the presence of a catalyst such as Ru catalyst and Ni catalyst. Nevertheless, as shown by the research of McCarty et al. (McCarty et al.: J. Chem. Phys. vol. 72, No. 12, 6332, 1980; J. Chem. Phys. vol. 74, No. 10, 5877, 1981), the sulfur adsorptivity of Ni and Ru is so powerful that the most part of the catalyst surface is covered by sulfur even if the sulfur content of the hydrocarbon is trace. Specifically, in the condition of about 0.1 ppm in the sulfur content, the best level at the present, about 90% of the surface of the Ni or Ru catalyst will be covered by sulfur in a short time under the inlet condition (about 450.degree. C.) of a conventional steam reforming process. This means that the present level of desulfurization of hydrocarbon cannot prevent the sulfur poisoning of the catalyst in the steam reforming.
To solve such problems, a steam reforming process which uses a hydrocarbon which has been desulfurized to less than 0.05 ppm is proposed in Japanese Patent Unexamined Publication No. 17003/1987. However, the process disclosed there cannot satisfactorily prevent the poisoning of the steam reforming catalyst because of insufficient desulfurization of the hydrocarbon, and cannot decrease the amount of steam used as described later.
On the other hand, when hydrocarbon such as naphtha is subjected to steam reforming in a reformer furnace, decomposition of higher hydrocarbons which is an endothermic reaction occurs at the inlet part of the reaction tube in the furnace. Since the reaction rate at the inlet part is determined by heat flux using a generally used reactor tube at the present, space velocity (SV) is limited even if a steam reforming catalyst of superior activity is applied. If the temperature of the inlet part was raised, higher hydrocarbon would be decomposed into carbon at the part.
It has been proposed that an adiabatic low-temperature steam reforming reactor (called "prereformer", hereinafter) be equipped in the upstream of an usual high-temperature steam reforming furnace (external heating reactor). In this system, decomposition of higher hydrocarbon is completed in the prereformer, and the size of the reformer furnace is reduced by raising the inlet temperature. However, the catalyst in the prereformer is easily poisoned by sulfur because relatively low temperature region is long in the prereformer, and so the catalyst bed is designed to be of enough volume. As a result, little improvement can be achieved by this system compared with a conventional single reformer furnace system.